The present invention relates to electrical braking systems for alternating current electric traction motors used in electrically propelled vehicles and, more particularly, to a braking effort control which can be used to hold a vehicle in a stopped position.
Electrical braking is used in various electrically propelled vehicles to supplement and reduce reliance on mechanical friction brake systems. Electric traction motor propelled vehicles such as locomotives, transit cars and off-highway vehicles are typically equipped with electrical retarding or braking systems. Such systems are designed to use the traction motors coupled in driving relationship with wheels of the vehicle in a power generation mode in which the rotation of the wheels drives the motors which are so energized as to act as electric generators. The power produced by the motors is often dissipated in high power resistors, generally referred to as a dynamic braking grid, with the magnitude of power being proportional to braking effort for any given speed.
An electric propulsion system for a traction vehicle, such as a locomotive, typically comprises a prime mover-driven synchronous electric generator or alternator for supplying electric power to a plurality of high horsepower electric traction motors respectively connected in driving relationship to the wheel/axle sets of the vehicle. The prime mover is commonly a diesel engine, and the traction motors are generally adjustable speed, reversible alternating current (AC) electric motors. A vehicle operator controls the vehicle speed and direction of travel, i.e., forward or reverse, by positioning of a speed control selector in one of a plurality of notch positions. This speed control selector is adapted to set motor power to establish a desired vehicle speed.
A typical AC electric motor propelled locomotive includes a rectifier circuit coupled to receive AC power from the alternator for converting it to DC power on a DC link. A DC to AC inverter is coupled between the DC link and each of the AC traction motors. The inverters can each be controlled to supply power to the respective motors so as to effect rotation of the motor rotors in either a clockwise or counter clockwise direction with corresponding forward or reverse movement of the locomotive. The inverters are typically constructed of a plurality of controllable electric switches such as thyristors (SCR), gate tum-off devices (GTO) and field effect transistors (FET). Each switch is controlled such as in a pulse width modulation (PWM) manner so as to regulate the effective AC frequency and current supplied to the motors.
During electrical braking of the locomotive, the AC motors are operated as generators with their respective rotors being mechanically driven by rotation of the locomotive wheels. The current generated by the motors during dynamic electrical braking is dissipated in dynamic braking grids coupled to the DC link. The magnitude of braking effort is a function of the current or electrical power produced by the motors and can be maintained at some maximum level until locomotive speed falls below a minimum value, typically about 2.5 miles per hour. Below that speed, the rotor rotational speed is not sufficient to maintain full braking effort and such effort is generally tapered toward zero. In some instances, dynamic braking terminates at some positive speed, such as 0.5 miles per hour, although the taper could be extended to about zero speed.
When the level of electrical dynamic braking is reduced, slowing and stopping of the locomotive relies on a conventional air operated mechanical braking system. The mechanical brakes are used not only to stop the locomotive but to hold it in position once stopped. The loading of the mechanical braking system promotes wear and tear and requires frequent maintenance. The air brake system requires time to recharge the reservoir after each brake application in order to again deliver full braking effort. Thus, there are at least two disadvantages to existing traction vehicle braking systems, namely that the level of available electrical braking drops off with reduced vehicle speed and that the mechanical braking system suffers excess wear and stress from extended use and the effect on air brake recharge. Accordingly, it would be advantageous to provide an electrical braking system which can provide sustained braking effort at zero speed. Also, stops are not smooth because of the large reduction in dynamic brake effort at low speed.